The present disclosure will refer to various documents, which, in order not to burden the reader, are listed at the end of the disclosure itself and are recalled in the body of the disclosure with the number with which they appear in said list indicated in square brackets, i.e. [x].
In numerical, or digital, transmission systems, a technique for transmitting the bit generated by the source envisages grouping said bits in complex symbols that identify the amplitude and the phase of the signal used for modulating a carrier. Typical examples of this technique are the modulations known as QAM (Quadrature Amplitude Modulation) and PSK (Phase-Shift Keying). The complex symbols of a QAM (or PSK) scheme can be associated with m bit. The modalities with which the bits are associated with the S=2m complex symbols is referred to as “mapping”, whilst the set of the symbols is referred to as “constellation”.
For instance, the QPSK (Quadrature Phase-Shift Keying) technique refer to 4 complex symbols that may correspond, respectively, to the pairs of bits 00, 01, 10 and 11. Gray mapping is an example of known technique in which two adjacent complex symbols represent sets of bits that differ from one another at the most by one bit. The complex symbols may be represented in the complex plane where the two axes represent, respectively, the in-phase (I) component and the quadrature (Q) component of the complex symbol.
For instance, FIG. 1 illustrates an example of a QPSK constellation, which can represent the bits via a Gray-mapping rule (dots), together with a possible received symbol (cross).
The numeric data (bits or symbols) are transmitted on physical channels, which normally alter them on account of the additive noise. In addition to this, in wireless systems the channel is subject to fading with consequent phenomena of distortion (variations of phase and amplitude). This means that the data received do not coincide with the ones transmitted, rendering necessary an action of equalization for estimating the transmitted data. The channel coefficients may be estimated prior to said equalization and be known by the equalizer. The “robustness” of a connection used for the transmission depends upon the capacity of the receiver to detect in a reliable way the transmitted bits (i.e., to detect the transmitted 1's as 1's and the transmitted 0's as 0's).
A widely used modulation technique in the wireless context is the technique known as OFDM (Orthogonal Frequency-Division Multiplexing). OFDM systems envisage dividing the total flow of information that is to be transmitted into various flows, each of which having a lower data rate and being designed to modulate in frequency a respective “subcarrier” of the main carrier. Equivalently, the total bandwidth is divided into as many sub-bands, each centred on one of the subcarriers. This operating mode renders the communication more robust in a context of wireless channel subject to multi-path fading and likewise simplifies the operations of equalization in frequency.
A sector of considerable potential interest for the use of said modulation techniques is constituted, among other things, by the applications known as vehicle-to-vehicle (or V2V) and vehicle to roadside-unit (or RSU), which is also known as vehicle-to-infrastructure (or V2I), applications. This applies in particular to the function of support of the applications regarding Intelligent Transportation Systems (ITS). Even though at the moment it does not exist an official classification of its applications, it is a very extensive set of applications that range from the improvement of the conditions of driver safety (which is a major purpose of said networks) to traffic control and to the reduction of traffic congestion, and to automatic toll collection.
For V2V and V2I applications, for which radiofrequency communications are required, an asserted standard is the one known as Dedicated Short-Range Communications (DSRC). This is a communication service for short or average distances that is able to support different applications (for example, public safety or else automatic toll collection) with a very low latency and a high data rate. As regards the physical layer (PHY) of DSRC technologies, the applications under study in various countries of the world are currently based prevalently upon the IEEE 802.11p standard [1], which is an extension of the IEEE 802.11 Wireless LAN (WLAN) specifications that is aimed at enabling the function known as Wireless Access in the Vehicular Environment (WAVE). The WAVE operating modes envisage data exchanges between vehicular devices in communication contexts that vary rapidly.
The IEEE 802.11p physical layer is very similar to current IEEE802.11 a/g standards, based upon an OFDM modulation, with the main difference represented by the use of the band around 5.9 GHz (5.85-5.925 GHz), instead of the 5.2-GHz band, and by a smaller bandwidth (10 MHz instead of 20 MHz).
In a physical layer (PHY) perspective, the application of a standard of an IEEE 802.11 type to mobile-communications systems comes up against some difficulties linked to the disturbance of the signal due to the radio channel.
For instance, phenomena of reflection and diffraction of the signal may give rise to multiple replicas of the transmitted signal, i.e., to multi-path phenomena. Each of said multi-path components may be characterized by a different phase and a different amplitude. The channel impulse response (CIR) in the discrete-time domain and the associated power-delay profile (PDP) enable representation of each multi-path contribution as a tap in the time domain. Each tap is typically expressed in the form of a complex value representing the respective contribution to the overall signal received, of which the modulus represents the associated level of intensity and the angle represents an associated phase rotation. The delay spread of the channel is the delay between the instant of arrival of the first multi-path contribution and the instant of arrival of the last multi-path contribution in the PDP. Frequently, a single value is used that takes into account each multi-path contribution in the form of a root-mean-square (RMS) time delay spread, which measures the dispersion of the delay around its mean value. The signal may be subjected to greater distortion by those channels that are characterized by a higher RMS delay spread. The time-domain multi-path effects admit of a dual representation in the frequency domain, where they determine the level of selectivity in frequency of the channel. This is measured via the coherence bandwidth, which is inversely proportional to the delay spread and represents the value of the frequency band in which the amplitude of the channel frequency response assumes an almost constant value.
As compared to the indoor propagation environment, the outdoor propagation environment is characterized in general by larger delay spreads. This in so far as the radio signal that propagates out of doors encounters obstacles, such as buildings and trees, that may give rise to multi-path effects due to reflection and/or diffraction that are located at greater distances from one another as compared to what occurs in indoor environments. As a result, the interference induced by the OFDM symbol transmitted previously, i.e., the inter-symbol interference (ISI) may arise in an outdoor environment much more frequently and intensely than in an indoor environment. An important characteristic of OFDM modulation is its capacity for mitigating multi-path effects through the inclusion of the guard interval (GI), i.e., a time interval purposely inserted between OFDM symbols transmitted in succession. If the GI is longer than the delay spread of the channel and the system is well synchronized, there is, theoretically, no inter-symbol interference. It should be noted that the reduction of bandwidth envisaged by IEEE 802.11p as compared to IEEE 802.11 a/g results in a longer duration of the GI that is able to cope with the major multi-path effect of urban environments.
Also the phase distortion in the frequency domain varies with time on account of the well-known Doppler shift, which derives from the relative movement between the transmitter and the receiver and is proportional to the speed of relative displacement. In non-line-of-sight (NLOS) configurations, on account of the presence of multiple reflections, the system becomes subject to multiple values of frequency shift, giving rise to the phenomenon known as Doppler spread. At the receiver, the Doppler spread can induce a considerable degradation of the performance, which may prove critical. The Doppler effect can also lead to a degradation of the orthogonality between the subcarriers, causing a single subcarrier to be subject to interference by the other subcarriers within one and the same OFDM symbol, thus giving rise to the phenomenon known as inter-carrier interference (ICI).
Furthermore, in OFDM systems designed for indoor use, the exposure to a multi-path propagation typical of an outdoor context can give rise, not only to inter-symbol interference, but also to greater difficulties in channel estimation. Moreover, mobility induces, within the duration of the individual packet, time-variable phenomena that may give rise to interference between the carriers such as to require techniques for tracking the variations of the channel after an initial estimate.
For these reasons, an advanced receiver designed to operate in an outdoor context and in conditions of mobility draws benefit, in terms of improvement of the performance, from the possibility of estimating/tracking the time-variant wireless channels with a frequency higher than what occurs for the static channels so as to take into account the presence of the Doppler effect. As illustrated in [2], improved techniques of channel estimate and tracking may represent an important factor for V2V/V2I receivers as compared to IEEE802.11 devices currently existing on the market.